<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 这句话也是我们css的第一行代码 */
    * {
      margin: 0;
      padding: 0;
    }
    span {
      background-color: #e1adad;
      /* padding: 20px; */
      margin: 20px;
    }
  </style>
</head>
<body>
  123
  <ul>
    <li>1</li>
    <li>3</li>
    <li>3</li>
  </ul>
  <span>行内元素尽量只设置左右的内边边距</span>
</body>
</html>